fix: reject premature EOF in file request bodies

[ryanfowler]

Summary

• reject short multipart file reads instead of emitting undersized bodies
• make multipart streams terminal after file open or read errors
• enforce captured lengths for ordinary file request bodies and signing/materialization paths
• add truncation and synthetic short-reader coverage

Validation

• cargo fmt --check
• cargo clippy --locked --all-targets --all-features -- -D warnings
• cargo test --locked --all-features --lib --bins (866 passed)